1. Field of the Invention
The invention relates to apparatus for curing resin material onto continuous webs of cloth or paper material. The invention particularly relates to the formation of coated abrasive materials and the curing of backing coats on cloth or paper which will be used for formation of coated abrasives.
2. Prior Practices
It is known in the coated abrasive art to apply binder and abrasive grains to a paper or cloth substrate which is cured yielding sufficient strength for the following applications, subsequently the size coat is applied and the product completely cured. Suitable binders are for example glutelin glue, phenolic resins and, if water proof papers are desired, polyurethane resins, epoxy resins and alkyd resins, possibly in combination with melamine resins. Special requirements as related to technique, apparatus and time are necessary for the curing process. To avoid destruction of the substrates usually consisting of polyester or cellulose, curing should be effected at a maximum temperature of 120.degree. to 130.degree. C. Rapid curing allowing for the use of a horizontal dryer is difficult, because of the formation of gas bubbles affecting the adhesion of the resin on the substrate. The drying of the coated material sufficient to be rolled for curing generally requires several hours, and is therefore carried out in a festoon oven. The festoon oven through which the coated web material is passing, enable a long drying or partial cure process, but there are also disadvantages, such as the formation of defects where the material is suspended, sagging of the binder and changing of the grain position due to the vertical suspension, variation of temperature and the resulting inconsistant crosslinking of the binder produced by the necessary slow air circulation. After removal from the festoon oven, it is then necessary to completely cure the rolls of partially cured abrasives by slowly heating in an auxiliary oven. Slow heating is necessary to prevent an uneven cure caused by widely different temperatures between the outside and inside of the rolls.
It is also known to produce abrasives by coating a substrate using a heat polymerizable curable synthetic resin as well as abrasive grains and by subsequently curing the applied layer by means of infrared radiation. The relatively long curing period of the synthetic resin is a disadvantage of this processing method. Owing to the long curing period and the elevated temperature the substrate is also strongly attacked. Furthermore the processing speed is low during the production of abrasives.
There are several disadvantages of the predominant commercial practice of forming coated abrasives. There are several curing steps in the typical process for formation of waterproof cloth-backed abrasives. The major areas of production may be considered as first the cloth treatment to prepare resin treated base cloth for application of abrasives and second the making of the coated abrasives using the previously prepared base cloth. The base cloth is coated with at least one backing coat of resin which impregnates the cloth with resin and fills interstices in the back of the cloth. The backing cloth is also coated with at least one face coat that fills interstices of the cloth on the side where abrasive grain is placed. The face coat(s) of the backing cloth also aids in adhesion of the coats containing the grains onto the cloth.
The second major area of coated abrasive formation is the drying or partial curing of the make coat which contains the grain and drying or partial cure of the size coat which is an overcoat placed onto the coated abrasive after the grain is at least partially cured and adhered onto the backing by the make coat. There may be pre-size coats prior to the make and size coats utilized in some instances. The partial curing of the make and size coats as set forth above generally is done in a lengthy festoon dryer that requires a tremendous amount of floor space and energy. Further, both the festoon and auxiliary ovens where the curing takes place over a long period are difficult to completely control for accurate temperature. There also is the problem of the resin and grain shifting positions during curing because of the long hang times in the partially cured or uncured form. Then after removal from the festoon oven, further energy is used in the oven treatment of the rolls to obtain complete cure.
It has been suggested in the U.S. Pat. No. 4,047,903 Hesse et al that the formation of coated abrasives be carried out with at least one layer of the resin being cured by electron beams. However, there has remained a need for apparatus which would allow the commercial exploitation of electron beam curing. Hesse et al does not set forth apparatus that would allow the continuous formation of coated abrasives. There are extensive difficulties in commercial exploitation of electron beam curing. The conventional electron beam units are not accessible for easy cleaning. The conventional units do not allow rapid adjustment for curing from either side of the web carrying the coated abrasive. The installations may be bulky with walls of cement about 3 feet thick. Further, the conventional electron beam units do not allow easy stringing of new web material into the machine for rapid changeovers from one material to another.
When forming coated abrasives, there may be required very thick coats of resin compared with prior uses of the electron beam. The resins necessary also are very sticky prior to being completely cured. Therefore, multiple path systems such as disclosed in some prior electron beam curing systems such as U.S. Pat. No. 3,022,543 are not satisfactory since if the resins touch a roller the system will gum-up and not perform. Another difficulty with the formation of abrasives with electron beam curing apparatus such as presently available is that in coated abrasive formation there is always a certain amount of abrasive grain which becomes detached from the coated abrasive during formation and can detrimentally affect the equipment if it is not possible to regularly clean and maintain the equipment. The equipment becomes contaminated by adhesive buildup and by material such as abrasive grit and dirt which becomes embedded in the abrasive. Another difficulty is that generally coated abrasives are made with multiple changes of grit size, backings and resin coatings. Therefore it is necessary to stop and start the system at relatively frequent intervals. Present systems of electron beam curing, designed for use in other arts, do not allow rapid cleaning and restringing of webs in the equipment. Therefore, if used for coated abrasives, the amount of up time would be so short as to not be economical.
Therefore there remains a need for apparatus which will allow formation of coated abrasives in a low-cost commercially satisfactory manner.